Various measurement devices for measuring electrical parameters (such as voltage, current and resistance) are widely used in industrial production and daily life. A multi-meter is a typical electrical measurement device that mainly measures an AC or DC voltage, current and resistance. A typical configuration of the multi-meter generally includes a pair of test lines, wherein one end of each test line is connected to an apparatus or a device under test, and the other end is inserted into a jack on a panel of the multi-meter. The device under test and a measurement circuit inside the multi-meter are therefore electrically connected with each other.
The typical multi-meter usually uses a single-ended amplifier as the pre-amplifier in its internal measurement circuit. In order to measure a test voltage, the multi-meter needs to perform two tests which are the high-side test and low-side test, to obtain two readings, and to calculate and present the value of the test voltage based on the corresponding readings. The high-side and low-side tests can reduce offset voltage disturbances of the amplifier on the measurement results. Furthermore, when a high-precision multi-meter is used to measure the resistance of the device under test, the multi-meter needs to perform four tests to accurately measure the resistance of the device under test. The device under test needs to be tested under two reverse test currents to generate four readings, which include two readings for the high-side test and the other two readings for the low-side test. The two reverse test currents can reduce a thermal noise or low-frequency noise (such as offset voltage-induced noise) in a signal chain of the measurement circuit. However, this measurement method is complicated.